The present invention relates to forming glass articles, and more particularly to a method of drawing glass fibers, sheets, ribbons or the like from a preform or source of glass such as an orifice.
Certain glasses such as halides, chalcogenides, oxy-halides, lead silicates, phosphates, borates and the like, which exhibit narrow working ranges, are difficult to form into fibers and thin sheets. Many of these glasses also exhibit low melting point temperatures. Some of these glasses readily devitrify, thereby requiring short residence time at elevated temperature to prevent devitrification. Glasses having narrow working ranges are exemplified by those taught in U.S. Pat. Nos. 4,314,031, 4,142,986, 4,405,724, 4,537,864, 4,668,641 and 4,752,593.
Such glasses have generated considerable interest in optical applications such as fibers, lasers and the like. Fluoride glasses, for example, are attractive candidates for transmission optical fibers, because of their low intrinsic scattering loss properties; fluoride glasses can also function as host materials for lasing dopants.
It has been difficult to draw components such as transmission optical fibers, fiber lasers and the like from preforms or melts of narrow working range glasses. Very precise temperature control is required in the fiber drawing apparatus. Radiation is not an effective heat transfer mechanism at the low draw temperatures of some of these glasses. Certain of the above-mentioned glasses devitrify if residence time at elevated temperature is too long.
When attempts have been made to draw narrow working range glasses by the double crucible method, control of glass flow has been a problem because of the sensitivity of viscosity on temperature.
In preform drawing apparatus, very sharp vertical temperature gradient is required between the root portion of the preform, which is held at draw temperature T.sub.d, and the adjacent portion, which is at the glass transition temperature T.sub.g. For example, in a tin fluorophosphate glass, the temperature difference between T.sub.g, where viscosity is about 10.sup.13 poise, and T.sub.d, where viscosity is about 10.sup.6 poise is about 50.degree. to 75.degree. C. If the drawing temperature is too high, the preform root melts and runs from the furnace; if it is too low, the fiber breaks. The magnitude of this allowable temperature interval where fiber drawing can occur may be only a few degrees.